1. Field of the Invention
This invention generally relates to lithography, and more specifically to the protection of lithographic reticles without the use of a pellicle.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A semiconductor wafer, for example, can be used as a substrate to fabricate an integrated circuit.
During lithography, a reticle is used to transfer a desired pattern onto a substrate. The reticle is formed of a material transparent to the lithographic wavelength being used, for example glass in the case of visible light. The reticle has an image printed on it. The size of the reticle is chosen for the specific system in which it is used. A reticle six inches by six inches and one-quarter inch thick may be used, for example. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer corresponding to the image printed on the reticle.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.
As should be clear from the above discussion, the accurate location and size of features produced through lithography is directly related to the precision and accuracy of the image projected onto the wafer.
The rigors of sub-100 nm lithography place stringent demands not only on the lithography tool, but also on the reticle. Air borne particles and dust that settle on the reticle can cause defects on the wafer. Small image distortions or displacements in the reticle plane can swamp critical dimension and overlay error budgets. The conventional solution is to use a thin piece of permanently fixed transparent material as a pellicle for the reticle. This pellicle remains in place during all stages of the lithography process. A pellicle has a dual role in improving the accuracy of the image projected onto a wafer. First, a pellicle serves to protect the reticle from direct contact with particulate contamination. Particles that settle on the reticle can produce image distortion and so must be removed. Removal of particles from the reticle can cause damage to the reticle because such removal may involve direct contact with the reticle. When a pellicle is used, particles will settle on the pellicle rather than the reticle. Thus, it is the pellicle which must be cleaned. Cleaning the pellicle rather than the reticle poses fewer dangers to the integrity of the reticle since the reticle is protected during this cleaning by the pellicle itself.
The second role played by a pellicle is related to the standoff of the pellicle. During exposure, the focal plane corresponds to the location of the image printed on the reticle. By including a pellicle, any particles in the system will settle on the pellicle rather than the reticle. By virtue of the thickness of the pellicle, and thus the distance between the surface of the pellicle and the patterned surface of the reticle, these particles will not be in the focal plane. Since the pellicle lifts the particles out of the focal plane, the probability that the image projected onto the substrate will include these particles is greatly reduced.
This solution discussed above works well in many conventional lithographic processing techniques. Since materials are available for producing transparent pellicles and reticles, the use of such a system is convenient in, for example, a system in which light must pass through both the reticle and the pellicle.
The pellicle approach, however, is not well suited for use in extreme-ultraviolet (EUV) applications. Currently, there are no materials sufficiently transparent to EUV that can be used to make a pellicle. In EUV lithography, the EUV does not pass through the reticle, but is reflected off the image side of the reticle. This technique is known as reflective lithography. If a pellicle were to be used in a reflective lithography process, the EUV would necessarily pass through the pellicle twice, once on the way to the reticle and again after reflecting off of the reticle. Thus, any amount of light loss associated with the pellicle is effectively doubled with EUV processing techniques.